The present invention relates to a signal detection circuit suitable for a radio paging receiver and, more particularly, to a circuit for detecting digital calling signals for use in a radio paging system.
In a radio paging system, a transition is now under way from tone signal processing to digital signal processing to meet the ever increasing amount of information and number of subscribers. Signal formats heretofore proposed for digital signal processing may be typified by the format called POCSAG (Post Office Code Standardization Advisory Group) codes proposed by BRITISH TELECOM of Britain. The signal according to the POCSAG system includes a preamble code signal followed by a plurality of batches which respectively indicate call numbers. Each batch begins with an SC (synchronization codeword) signal adapted to achieve frame synchronization and includes an address codeword signal following the SC signal.
When a radio paging receiver detects a preamble code signal while operating in an intermittent reception mode, which is usually referred to as a battery saving function, the battery saving function is temporarily suspended. Then, the receiver is continuously supplied with power for a certain period of time (such as 1.5 seconds). As the receiver receives the subsequent SC signal, the battery saving function is entirely cancelled in order to detect an address codeword signal which will follow the SC signal. For details of the battery saving function, refer to the U.K. Patent Application published on Oct. 7, 1981 under Ser. No. 2,072,908 corresponding to Japanese patent application No. 55-39038, which was assigned to the same assignee as in this application.
As described, the SC signal is adapted for frame synchronization and complete suspension of the battery saving function. Many kinds of SC signals may be used to accommodate a larger number of addresses, as has been contemplated in the art concerned. Generally, an SC code signal is constituted by an error correcting code in order to increase the reliability of the communication system.
However, a circuit used for detecting the signal format including error correcting codes has various problems. For example, if the beginning or head end of the SC signal is unidentifiable, the SC signal cannot be correctly detected. Therefore, it is necessary to add a circuit for detecting the beginning of the SC signal, complicating the circuit structure as a whole.